1. Field
The disclosed concept relates to vacuum switching apparatus such as, for example, vacuum switches including a vacuum envelope such as, for example, vacuum interrupters. The disclosed concept also pertains to electrode assemblies for vacuum interrupters.
2. Background Information
Vacuum interrupters include separable main contacts disposed within an insulated and hermetically sealed vacuum chamber. The vacuum chamber typically includes, for example and without limitation, a number of sections of ceramics (e.g., without limitation, a number of tubular ceramic portions) for electrical insulation capped by a number of end members (e.g., without limitation, metal components, such as metal end plates; end caps; seal cups) to form an envelope in which a partial vacuum may be drawn. The example ceramic section is typically cylindrical; however, other suitable cross-sectional shapes may be used. Two end members are typically employed. Where there are multiple ceramic sections, an internal center shield is disposed between the example ceramic sections.
Vacuum electrical switching apparatus, such as vacuum circuit interrupters (e.g., without limitation, vacuum circuit breakers; vacuum switches; load break switches), provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, and low level voltage conditions. Typically, vacuum circuit interrupters include a spring-powered or other suitable operating mechanism, which opens electrical contacts inside a number of vacuum interrupters to interrupt the current flowing through the conductors in an electrical system in response to abnormal conditions.
The main contacts of vacuum interrupters are electrically connected to an external circuit to be protected by the vacuum circuit interrupter by electrode stems, typically an elongated member made from high purity copper. Generally, one of the contacts is fixed relative to the vacuum chamber as well as to the external circuit. The fixed contact is mounted in the vacuum envelope on a first electrode extending through one end member. The other contact is movable relative to the vacuum envelope. The movable contact is mounted on a movable electrode axially slidable through the other end member. The movable contact is driven by the operating mechanism and the motion of the operating mechanism is transferred inside the vacuum envelope by a coupling that includes a sealed metallic bellows. The fixed and movable contacts form a pair of separable contacts which are opened and closed by movement of the movable electrode in response to the operating mechanism located outside of the vacuum envelope. The electrodes, end members, bellows, ceramic shell(s), and the internal shield, if any, are joined together to form the vacuum interrupter (VI) capable of maintaining a partial vacuum at a suitable level for an extended period of time.
The vacuum interrupter is only actively called upon, in abnormal conditions, to interrupt the fault current by opening the movable contact from the fixed contact. The majority of the time the vacuum interrupter is in the closed position with the movable contact in electrical connection with the fixed contact, passively passing the rated (i.e., normal) circuit current continuously. Due to the inherent electrical resistance of the vacuum interrupter itself, the passing of the continuous current generates heat, leading to a rise in the temperature of the components of the vacuum interrupter as well as the bus bars connected to the vacuum interrupter.
With the wide acceptance of vacuum interruption technology in medium voltage switchgear, vacuum interrupters are being used in more and more demanding applications. One example is the ever increasing continuous current requirement. As a result, the diameter of the electrode stems are becoming bigger and bigger. However, for an electrode with a diameter larger than about 2 inches, for example, the alternative current (AC) resistance, for the practical 50 Hz or 60 Hz currents, is significantly larger than its direct current (DC) resistance, due to skin effect and proximity effect. The size of the vacuum interrupter limits the diameter of the electrodes that can be fitted into it. For this reason, it is difficult to achieve a relatively high continuous current carrying capability of a given vacuum interrupter size.
There is, therefore, room for improvement in vacuum switches, such as vacuum interrupters, and in electrode assemblies therefor.